Some offshore units of the self-elevating type, referred to as “jack-up”, comprise:                a shell, allowing the unit to move by floating and receiving the useful part, and        a plurality of mobile legs, maneuverable in lifting or lowering directions along the shell, intended to come in rest on the ground.        
That way, these offshore units can move by floating when their legs are lifted up, while having a rest on the sea floor when these legs are lowered down.
To ensure their manoeuvre in lifting and lowering directions, these legs conventionally include racks cooperating with pinions driven by motor means equipping the shell.
However, today, such a self-elevating offshore unit can rest its legs against the sea floor only in the presence of a small swell, i.e. typically waves lower than 2 meters high.
Indeed, in case of moderate to high swell (typically waves whose height is higher than 2 meters or higher than 4 meters, respectively), the roll and pitch movements of the floating shell, combined with the stiffness of the legs, cause dynamic overloads at each percussion of these latter against the sea floor.
Such overloads come in particular from the pinion motor means that, due to their inertia, are not able to efficiently absorb the leg movements imparted by the swell.
This phenomenon is a serious limitation to the exploitation of such self-elevating units, used for example for the installation of wind turbines and also within the framework of certain oil drillings.
To remedy these drawbacks, the pinions of certain structures are mounted on a floating frame and connected to the shell by an elastic device of the elastomeric cushion type.
Such an elastic device allows a partial reduction of the dynamic overload.
However, in practice, this compensation is not sufficient.
It also reduces the stiffness and the stability of the offshore unit in lifted position, which goes against the desired objective, i.e. mounting this offshore unit on legs resting on the sea floor in order to immobilize it.
There hence exists a need for a maneuvering device adapted to efficiently dissipate and absorb the efforts coming from the percussion of the mobile legs against the sea floor.
Such a structure would be particularly interesting to allow the manoeuvre of the legs of a self-elevating unit in usually unfavourable swell conditions (i.e. moderate, or even high swell).